This invention relates to an optical system in which at least one element is a so-called gradient glass having a radial difference in refractive index.
The correction of the image defects of an optical system is carried out by suitable combinations of different lenses which differ with respect to their index of refraction, their radii, and their thichnesses. In addition to the use of aspherical surfaces it is also known to use lenses which have an index of refraction which is dependent on their position.
In West German Auslegeschrift (provisional patent) 1,913,358, optical elements having a radial difference in refractive index as well as methods for the manufacture thereof are described. This auslegeschrift relates to light-conducting optical glass bodies, and discloses a method with which, by means of an ion-exchange transverse to the direction of the passage of light through the light guide, the concentration of at least two types of cations change in opposite direction over the entire cross section of the glass body. As a result, the indices of refraction continuously decrease or increase respectively in said direction, so that the light path is curved in the direction of increasing index of refraction.
Furthermore, in West German Offenlegungsschrift (published patent application) 1,939,478 there is disclosed a lens element which consists of a translucent body having a distribution of the index of refraction substantially in accord with the relationship EQU n=N(1.+-.ar.sup.2)
where N is the index of refraction in the center of a cross sectional surface perpendicular to the central axis of the body, n is the index of refraction at a radial distance r from the center, and a is a positive constant. The published application utilizes the fact that translucent materials having the above indicated distribution of the index of refraction have a lens effect, and mentions as one example for the use of the invention a microscope in which such a lens element with radial difference in refractive index is used as an objective. No information is given as to the image-forming qualities or the aberration correction of such a microscope.